Evaluates to the current definition of ${macro},
after substituting the string s1 for every occurrence of s2
that occurs either immediately before a blank or tab, or at the end of
the macro definition.

Sets the shell that interprets commands. If this macro isn't defined in the
description file, the value depends on your system.
Some UNIX implementations use the shell from the user's environment (as with
other macros). Other implementations (including SVR4) set the default
SHELL to /bin/sh.

VPATH

(Not available in all variants of make.)
Specifies a list of directories to search for prerequisites
when they aren't found in the current directory.